Monitoring apparatuses installed outdoors are mainly used for monitoring invasion of suspicious individuals, cars, and the like. As such a method of sensing suspicious individuals or the like, a method of calculating a difference from a background image is known. It is desirable that the background image used at this time be an image from which an image with a moving object has been removed. For example, at the time of rain, an image from which raindrops have been removed is used. Therefore, a system that performs image processing in order to remove rain, snow, and the like reflected in an image has been proposed (for example, refer to Patent Document 1).